The present invention relates to a method for purifying matter such as soil, sediment, sludge and water contaminated with halogenated organic compounds, particularly a chlorinated organic compound. The present invention particularly relates to a method for purifying contaminated matter by reductive dehalogenation combining a chemical reaction with a biological reaction, thereby decomposing the halogenated organic compound.
Recently, halogenated organic compounds such as tetrachloroethylene, trichloroethylene, 1,1,1-trichloroethane, and dichloroethylene are wide used as a degreasing agent for electronic components and mechanical metal components and a cleaning agent for dry cleaning. Halogenated organic compounds are contaminants in soil and ground water. These halogenated organic compounds do not readily decompose in the natural world and are hardly soluble in water, and therefore tend to accumulate in soil and to penetrate into ground water. Moreover, halogenated organic compounds are known to induce hepatic disorders and cancer. Therefore, it is desirable to decompose halogenated organic compounds such as chlorinated organic compounds in soil and so on.
In these days, bioremediation has been receiving attention to purify soil, ground water and so on contaminated with halogenated organic compounds. Bioremediation is safe and has improved costs to effects. However, the bioremediation requires a long period of time to be effective and there are limits on the kinds and concentrations of substances that can be decomposed. Therefore, we may not necessarily be satisfied with the bioremediation
The bioremediation includes aerobic decomposition of trichloroethylene with methane assimilating microorganism, toluene/phenol decomposing microorganism, ammonia oxidizing microorganism and alkene assimilating microorganism, and there are numerous reports on the aerobic decomposition. However, the aerobic decomposition has disadvantages as follows: decomposition reactions are unstable; the range of substances able to be decomposed is very limited; and highly chlorinated compounds such as tetrachloroethylene and carbon tetrachloride cannot be decomposed.
On the other hand, many anaerobic microorganisms have specificity to decompose a wide range of highly chlorinated compounds such as tetrachloroethylene, trichloroethylene, carbon tetrachloride and so on. However, the anaerobic microorganisms have disadvantages in that growth thereof is very slow; and anaerobic microorganisms produce and thus accumulate strongly toxic intermediate metabolites in the process of the anaerobic decomposition (see H. Uchiyama and S. Yagi; Bioscience and industry, Vol. 52, No. 11, pp. 879-884, 1994).
On the other hand, it has been reported that halogenated organic compounds can be decomposed by chemical reactions, and reductive treatment of chlorinated organic compounds with metallic iron has been disclosed (see T. Senzaki; Treatment of Ground Water Contaminated with Chlorinated Organic Compoundsxe2x80x94treatment technique with activated carbon carrying metal iron at low temperatures; xe2x80x9cPPMxe2x80x9d Vol. 26, No. 5, pp. 64-70, 1995). In view of the foregoing, the present inventors tried dechlorination tests wherein metallic iron is added to soil in the absence of a carbon source for a microorganism. However, under conditions that microorganism is not cultivated and particularly conditions that a reductive atmosphere and a neutral condition are not maintained, the present inventors did not observe any dechlorination reaction. Moreover, the addition of an iron salt such as FeCl2, FeCl3 and FeSO4 instead of the metallic iron did not produce the dechlorination reaction, either.
It has been reported that metallic iron and high-pressure air can be injected into soil for reacting iron powder with halogenated organic compounds in the soil to convert into inorganic compounds, thereby detoxifying the same (see Japanese Patent Application Laid Open No. 8-257570). However, the method has disadvantages concerning equipment for injecting air and there is a chance that the halogenated organic compounds may diffuse. Moreover, the use of high-pressure air increases costs, and therefore is not practical.
It has been reported that chlorinated organic compounds contaminating soil and ground water can be removed by combining a natural compound having catalytic activity for dehalogenation with microorganism treatment (see xe2x80x9cNikkei Biotechxe2x80x9d published by Nikkei BP Inc., Oct. 7, 1996, No. 361, pp. 14-15). However, the document does not disclose specific natural compounds and the microorganism at all.
U.S. Pat. No. 5,411,664 discloses a method for decomposing halogenated organic compounds by adding fibrous organic matter and multivalent metal particles to a contaminated matter. However, the U.S. patent does not disclose a reducing agent such as reduced iron, cast iron, alloy, a water soluble reducing agent and so on. Moreover, the U.S. patent does not disclose maintaining the contaminated matter in a reductive atmosphere or a specific pH subsequent to adding the reducing agent.
Moreover, depending on the composition of a nutritional source added, biological reductive reactions such as sulfate reduction and methane fermentation may occur, and sulfur-containing noxious gases such as hydrogen sulfide and mercaptan are produced and a combustible gas such as a methane gas may be generated. Moreover, the production of iron sulfide may change color of soil into black. A reaction of metallic powder and water may produce a combustible hydrogen gas.
In a laboratory scale, it is easy to homogeneously mix a reducing agent and a nutritional source with a contaminated matter. However, in order to purify contaminated matter such as soil, in reality, a large amount of reducing agent and the nutritional source would be required, which may warrant engineering works. Moreover, it is not easy to homogeneously mix the contaminated matter in such a large scale. Furthermore, it is expected that mixing conditions may affect a decomposition rate of halogenated organic compounds. Particularly, a special technique is required to purify the contaminated matter having a volume of not less than 1 m3 and particularly not less than 10 m3.
It is an object of the present invention to provide a method for purifying matter contaminated with a halogenated organic compound by combining chemical reaction with a biological reaction, thereby decomposing the halogenated organic compound.
The first aspect of the present invention has an object of decomposing the halogenated organic compounds by reductive dehalogenation reaction.
The second aspect of the present invention has an object of decomposing the halogenated organic compounds by a chemical dehalogenation reaction.
The third aspect of the present invention has an object of preventing the generation of sulfur-containing noxious gas and combustible gas as well as the excessive decoloration of contaminated matter such as soil, which may occur as a result of by-products of the reductive dehalogenation reaction.
The fourth aspect of the present invention has an object of mixing the reducing agent and the nutritional source with the contaminated matter when the contaminated matter has a large volume.
The first aspect of the present invention is characterized by adding a prescribed reducing agent, thereby promoting the reductive dehalogenation reaction.
The second aspect of the present invention is characterized by adding a prescribed reducing agent, thereby promoting the chemical reductive dehalogenation reaction. Contrary to the first aspect of the present invention, the second aspect need not involve the use of a biological reaction. Use of the prescribed reducing agent allows to decompose halogenated organic compounds solely by chemical reactions.
In the third aspect of the present invention, a nutritional source containing an organic carbon and 20 to 50 percent by weight, based on the organic carbon, of an oxidized form of nitrogen is used so as to change a group of microorganisms involved in reductive dehalogenation reaction, thereby preventing soil from being changed in color to black and noxious gases such as mercaptan from being generated.
The fourth aspect of the present invention is achieved by mixing a reducing agent and a nutritional liquid with a contaminated matter, thereby uniformly mixing thereof, especially when the contaminated matter having a volume of not less than 1 m3 and particularly not less than 10 m3 is purified.
In the third and fourth aspects of the present invention, the reducing agent used in the first and second aspects of the present invention is preferably used but not limited thereto.
According to the first aspect of the present invention, there is provided a method for purifying matter contaminated with a halogenated organic compound, which method comprises the steps of: adding a reducing agent and a nutritional source for a heterotrophic anaerobic microorganism to the contaminated matter, the reducing agent having a standard electrode potential ranging from 130 mV to xe2x88x922400 mV at 25xc2x0 C. with respect to the standard hydrogen electrode, the reducing agent is at least one species selected from the group consisting of reduced iron, cast iron, iron-silicon alloy, titanium alloy, zinc alloy, manganese alloy, aluminum alloy, magnesium alloy, calcium alloy and a water soluble compound. The presence of such reducing agent promotes reductive dehalogenation reaction by combination of chemical reactions and microorganisms.
In the present invention, preferably, the reducing agent has the standard electrode potential ranging from xe2x88x92400 mV to xe2x88x922400 mV at 25xc2x0 C. with respect to the standard hydrogen electrode, and the reducing agent is at least one species selected from the group consisting of the reduced iron, the cast iron, the iron-silicon alloy, the titanium alloy, the zinc alloy, the manganese alloy, the aluminum alloy, the magnesium alloy, and the calcium alloy. Preferably, the reducing agent comprises the reduced iron. Alternatively, the reducing agent may comprise the cast iron. Alternatively, the reducing agent may be selected from the group consisting of the iron-silicon alloy, titanium-silicon alloy, titanium-aluminum alloy, zinc-aluminum alloy, manganese-magnesium alloy, aluminum-zinc-calcium alloy, aluminum-tin alloy, aluminum-silicon alloy, magnesium-manganese alloy and calcium-silicon alloy.
Preferably, the reducing agent is a water soluble compound. Further preferably, the reducing agent is an organic acid or derivative thereof, hypophosphorous acid or derivative thereof, or a sulfide salt.
Preferably, the reducing agent is a powder having a diameter up to 500 xcexcm. Preferably, the contaminated matter has a water content of at least 25 percent by weight.
Preferably, the method further comprises the step of maintaining the contaminated matter in a pH ranging from 4.5 to 9.0 subsequent to the adding step. Preferably, further comprising the step of maintaining the contaminated matter in a pH ranging from 4.5 to 9.0 under a reducing atmosphere subsequent to the adding step.
Preferably, the method further comprises the steps of adding an organic compost, a compostable organic material, a waste water containing organic matter or a waste containing organic matter to the contaminated matter and mixing thereof.
In the first aspect of the present invention, preferably, the water soluble reducing agent is monocarboxylic acid, dicarboxylic acid, tricarboxylic acid, tetracarboxylic acid or salt thereof, which may have 1 to 20 carbon atoms, and which may be substituted by a hydroxy radical; polyhydroxyaryl; or hypophosphorous acid or salt thereof. Preferably, the water soluble reducing agent is hypophosphorous acid or salt thereof. The reducing agent may be a salt made of the organic acid or the hypophosphorous acid and iron, titanium, zinc, manganese, aluminum, or magnesium.
Preferably, the method further comprises the step of adding at least one of an alkali metal compound and an alkaline earth metal compound to the contaminated matter for adjusting pH thereof. In the maintenance step, preferably, the halogenated organic compound is converted into an organic compound being free of a halogen atom. In the maintenance step, preferably, the halogenated organic compound is converted into a hydrocarbon being free of a halogen atom.
According to the second aspect of the present invention, there is provided a method for purifying matter contaminated with a halogenated organic compound, which method comprises the step of: adding a reducing agent to the contaminated matter, the reducing agent having a standard electrode potential ranging from 130 mV to xe2x88x922400 mV at 25xc2x0 C. with respect to the standard hydrogen electrode, the reducing agent is at least one species selected from the group consisting of reduced iron, cast iron, iron-silicon alloy, titanium alloy, zinc alloy, manganese alloy, aluminum alloy, magnesium alloy, calcium alloy and a water soluble compound.
In the second aspect of the present invention, preferably, the reducing agent has the standard electrode potential ranging from xe2x88x92445 mV to xe2x88x922400 mV at 25xc2x0 C. with respect to the standard hydrogen electrode, and the reducing agent is at least one species selected from the group consisting of the iron-silicon alloy, the titanium alloy, the zinc alloy, the manganese alloy, the aluminum alloy, the magnesium alloy, and the calcium alloy.
Preferably, the contaminated matter comprises 0.1 g to 100 g of an iron compound based on 1 kg of a dry weight of the contaminated matter. Further preferably, the contaminated matter comprises 1 g to 100 g of an iron compound based on 1 kg of a dry weight of the contaminated matter, and the iron compound comprises iron hydroxide (Fe(OH)3) or triiron tetraoxide (Fe3O4). Preferably, the reducing agent is at least one species selected from the group consisting of the iron-silicon alloy, titanium-silicon alloy, titanium-aluminum alloy, zinc-aluminum alloy, manganese-magnesium alloy, aluminum-zinc-calcium alloy, aluminum-tin alloy, aluminum-silicon alloy, magnesium-manganese alloy and calcium-silicon alloy.
Alternatively, the reducing agent preferably may be a water soluble compound. Preferably, the reducing agent is an organic acid or derivative thereof, hypophosphorous acid or derivative thereof, or a sulfide salt. Preferably, the reducing agent is a powder having a diameter up to 500 xcexcm.
According to the third aspect of the present invention, there is provided a method for purifying matter contaminated with a halogenated organic compound, which method comprises the step of: adding a reducing agent and a nutritional source for a heterotrophic anaerobic microorganism to the contaminated matter, the reducing agent having a standard electrode potential ranging from 130 mV to xe2x88x922400 mV at 25xc2x0 C. with respect to the standard hydrogen electrode, the nutritional source containing an organic carbon and 20 to 50 percent by weight, based on the organic carbon, of an oxidized form of nitrogen.
Preferably, the nutritional source contains 20 to 30 percent by weight, based on the organic carbon, of the oxidized form of nitrogen. Preferably, the organic carbon is supplied as a water soluble organic carbon source.
Preferably, the reducing agent is a metal having a standard electrode potential ranging from xe2x88x92400 mV to xe2x88x922400 mV at 25xc2x0 C. with respect to the standard hydrogen electrode. Preferably, the reducing agent is at least one species selected from the group consisting of reduced iron, cast iron, iron-silicon alloy, titanium alloy, zinc alloy, manganese alloy, aluminum alloy, magnesium alloy, and calcium alloy.
Preferably, the reducing agent is a water soluble compound. Preferably, the reducing agent is a powder having a diameter up to 500 xcexcm.
In the third aspect of the present invention, preferably, the oxidized form of nitrogen is in a form of a nitrate salt. Preferably, the nitrate salt contains alkali metal nitrate, alkaline earth metal nitrate, iron nitrate, titanium nitrate, zinc nitrate, manganese nitrate, aluminum nitrate or magnesium nitrate. Further preferably, the nitrate salt contains sodium nitrate, potassium nitrate or calcium nitrate.
Preferably, the organic carbon source is a sugar, an organic acid or derivative thereof, lower alcohol, a morasses waste, a liquor waste or a mixture thereof.
According to the fourth aspect of the present invention, there is provided a method of purifying a contaminated matter containing a halogenated compound and a solid matter, which method comprises the step of: mixing a reducing agent and a nutritional liquid containing a nutritional source for a heterotrophic anaerobic microorganism and water with the contaminated matter, the reducing agent having a standard electrode potential ranging from 130 mV to xe2x88x922400 mV at 25xc2x0 C. with respect to the standard hydrogen electrode, wherein the mixing step including a step of adjusting the contaminated matter at pH ranging from 4.5 to 9.0; and keeping the mixture in a condition that air hardly penetrates through a matrix.
In the present invention, preferably, the reducing agent may be in a powder form and wherein the nutritional liquid is added to the contaminated matter and mixed thereof, and then the reducing agent is added to the resultant mixture and further mixed thereof.
Preferably, the reducing agent is a powder having a diameter up to 500 xcexcm.
Preferably, the reducing agent is at least one species selected from the group consisting of reduced iron, cast iron, iron-silicon alloy, titanium alloy, zinc alloy, manganese alloy, aluminum alloy, magnesium alloy and calcium alloy.
Preferably, 1 to 10 percent by volume, based on the contaminated matter, of the nutritional liquid is added to the contaminated matter and mixed thereof as a first step; and then an amount larger than the amount of the first step of the nutritional liquid is added to the contaminated matter and mixed thereof as a second step.
Alternatively, 1 to 5 percent by volume, based on the contaminated matter, of the nutritional liquid may be added to the contaminated matter and mixed thereof as a first step; the nutritional liquid may be added to the contaminated matter and mixed thereof as a second step wherein a sum of the nutritional liquids added in the first step and the second step amounts 5 to 10 percent by volume, based on the contaminated matter, of the contaminated liquid; and the nutritional liquid is added to the contaminated matter and mixed thereof as a third step wherein an amount of the nutritional liquid added in the third step is more than an amount of the nutritional liquid added in either the first step or the second step.
Preferably, the reducing agent is a water soluble compound, and the reducing agent is dissolved in the nutritional liquid. Preferably, in the keeping step the mixture is kept at a temperature ranging from 17xc2x0 C. to 60xc2x0 C. for at least initial three days.
In the fourth aspect of the present invention, preferably, in overall, 15 to 25 percent by volume, based on the contaminated matter, of the nutritional liquid is added to the contaminated matter.
Preferably, in the keeping step the mixture is kept at a temperature ranging from 17xc2x0 C. to 60xc2x0 C. for at least initial five days. Further preferably, in the keeping step the mixture is kept at a temperature ranging from 20xc2x0 C. to 40xc2x0 C. for at least initial three, preferably five days.
Preferably, the keeping step is conducted in a situation that the mixture is separated from the environment. Preferably, the contaminated matter is covered by a material that does not penetrate air so as to maintain a condition that air hardly penetrates through a matrix. Alternatively, the contaminated matter is immersed in an aqueous liquid so as maintain a condition that air hardly penetrates through a matrix. Preferably, the nutritional liquid, the reducing agent and the contaminated matter are mixed in a container.